1. Field of the Invention
The subject matter of this invention is related generally to electromagnetic contactors and more specifically to apparatus for controlling an electromagnetic contactor.
2. Description of the Prior Art
Electromagnetic contactors are well known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is called a motor controller. A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. Disposed on the armature is a set of bridging contacts, the complements of which are fixedly disposed within the contactor case for being engaged thereby as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are usually interconnected with the fixed contacts and become interconnected with each other as the bridging contacts make with the fixed contacts. Generally, as the armature is accelerated towards the magnet, it must overcome two spring forces. The first spring force is provided by a kickout spring which is subsequently utilized to disengage the contacts by moving the armature in the opposite direction when the power applied to the coil has been removed. This occurs as the contacts are opened. The other spring force is provided by a contact spring which begins to compress as the bridging contacts abut the fixed contacts, but while the armature continues to move towards the fixed magnet as the air gap is reduced to zero. The force of the contact spring determines the amount of electrical current which can be carried by the closed contacts, and furthermore determines how much contact wear is tolerable as repeated operation of the contactor occurs. It is usually desirous for the contact spring to be as forceful as possible, thus increasing the current-carrying capability of the contactor and increasing the capability to adapt for contact wear. However, since this force must be overcome by the energy provided to the electromagnet during the closing operation, more closing energy will generally be required for relatively stiffer contact springs than for less stiff contact springs. Many electromagnets in contactors are powered by alternating current. However, no teaching has been found in the prior art for controlling the alternating current in a logical way to obtain predictable results. It would be desirable to control the alternating current in such a manner.